Shifting in a multiple speed ratio motor vehicle automatic transmission is generally initiated in response to the detection of predefined vehicle speed and engine load conditions, referred to collectively as a shift pattern. In a typical application, upshift and downshift vehicle speed thresholds are determined as a function of an engine load indicator, such as throttle position. An upshift to a higher output/input speed ratio is initiated if the actual vehicle speed exceeds the upshift threshold, while a downshift to a lower output/input speed ratio is initiated if the actual vehicle speed is lower than the downshift threshold.
The shift pattern data is empirically determined to achieve various fuel economy and performance requirements under "normal" road load conditions. In an electronic control application, the data is stored in a non-volatile memory array for subsequent retrieval by a powertrain control processor. A drawback of this approach is that the shift pattern developed for "normal" road load may not be appropriate when the vehicle is operated under heavy load conditions. Thus, it is not unusual for several upshifts and downshifts to occur in the course of climbing a steep grade, or while climbing a modest grade at high altitude or with a trailer attached to the vehicle. The successive upshifting and downshifting is not only annoying, but also causes excessive wear and heating of the transmission friction elements.
It has been proposed to modify the normal shift pattern in relation to the amount by which the grade load exceeds normal road load, but such modifications can only reduce the amount of shifting. Also, it has been proposed to make upshifting subject to a determination that the demanded output torque can be met in the upshifted speed ratio, but this is only effective in steady state driving conditions.